There was a long period where I was a fanatic for the Souls games, from Demon's to the Dark trilogy and even Bloodborne. During that period there was a distinct way everyone noticed how Bamco, the publishers for the Dark Souls games, and Sony, the rights-holders of Demon's Souls and Bloodborne, handled these games, whether it be budget, creative freedom for From Software, and of course marketing. Among other things. If you look at the most recent trailers released for Dark Souls 3 and their DLCs (my recommendation if you haven't played the game or the DLCs and may wish to play them some day: DON'T), they literally spoil everything in those games. It's been noted that every single boss, including the "secret" ones, appear prominently in the trailer for Dark Souls 3, the main game. Some are seen in more split-second cuts than others, but for the most part, it shows you pretty much everything. The way they showed off Nameless King was downright shameless. Then some teasers to lead up to each of the DLCs, until the final trailers before release which, you guessed it, gives away everything, including bosses. I thankfully stayed away from the last trailer for The Ringed City and it was all the better for it. When I decided to watch it after the fact I was just shaking my head like "Bamco, the hell is wrong with you."

Compare that to Sony's trailer for The Old Hunters, the highly touted, and rightly so, DLC for Bloodborne: https://www.youtube.com/watch?v=vbGdIdHOO3I

It's a great trailer because it gives you just tantalizing glimpses of a few of the areas you'll be traversing without revealing too much of anything. The DLC also includes 5 bosses and we see briefly 1 of them and the face of a second one. Most of the trailer is a showcase for some of the new weapons you'll get to play with by getting the DLC (all the combat shown is in areas in the main game). The use of Valtr's dialogue to "narrate" the DLC (drowned out a bit by the music, but ah what glorious music it is - another good selling point - that's the well-loved theme for one of the DLC bosses), they did right here what Bamco repeatedly failed with their trailers for everything related to Dark Souls 3. The trailer for the BB DLC treads that fine line between revealing enough to make you want to play it while holding enough back so you still aren't quite sure what you're getting into when going in.

Actual gameplay.

These options won't work for every game.

The HUD is often a vital part of the game; this is especially true in an RPG, where it is necessary to keep track of the state of your party, and you also need to execute commands for each character. Furthermore, in some RPGs (like the Dragon Quest series, for example), it is on the HUD that the action takes place; the game needs somewhere to display things like the actions taken, the damage dealt by an attack, and the fact that the party's Goof-off decided to take a nap in the middle of the battle. Also, in a visual novel that doesn't have voice acting, how will you present the text?

In some genres, difficulty can't be tweaked except by redesigning the game; in other words, you would need to have a whole new set of levels for each difficulty setting. This is an issue in puzzle games with fixed puzzles (in other words, something like Sokoban, not like Tetris), as well as in pure adventure games and visual novels.

As for controls, customizable controls work in most games, but even then I can find cases where that doesn't make sense. One example would be Jill Off with One Hand, in which every key on the keyboard does the same thing (allowing the game to be played with just one finger). Another example would be kinetic novels (if you count them as games, at least for the sake of this discussion).

Thanks everyone for the replies!
The game is first and foremost a shoot'em'up (like Raiden, Tyrian, etc.) but has RPG elements such as levelling, skill points, inventory/equipment systems (you can even change your ship)...with pixel art graphics and chiptunes music.

Here is a trailer i made a long time ago (when we released an alpha demo):

https://www.youtube.com/watch?v=2KTfHwhVRgY

Obviously, i made mistakes with that one (first, the logos at the start) and didn't have any sound for the game yet (only music). So i'll take your advice to make a better one for the release trailer. I have much more gameplay and fights to show so it should allow more diversity in the gameplay parts.
But aside from this, what do you think of it ?

Here's another one, a "cinematic" one.
https://www.youtube.com/watch?v=qm4lhdrkGzk

About controls, hud, etc. , yes i believe customizable controls is a must, especially on PC/Mac. So we do have it in the game.
@Kaboro: i might make one as a "third" trailer. It's a good idea.

Thanks again !

I watched it on my phone, without the sound.

My impression was that the first introductory cutscene was a tad too long. I had "registered" what was going on (ship, other ship, sentence), was wondering why it was lingering so much on those images, then a couple of more static images had me going "oh dear, more intro, and i still don't even knoe the genre of the game". Fortunately, the gameplay footage arrived right as my eyes were starting to roll upwards. But I still think that the intro could be shortened a lot. We "get it" fast enough, it overstays the provided info too much.

But again, I watched it without the sound. So maybe there are musical reasons for this pace, and maybe with the sound on things seem to go faster ?

Still, maybe testing it with sound off could be a good trick to check the pace.

Also maybe ending on a card summarizing the specificities of your game. Either inventive gimmicks or faithful classicism.

Thanks!
You're talking about the cinematic one, right ?
Good feedback, i'll keep it in mind when making the new one.

My goal is to make the "regular" trailer right now, i'll probably make 2 to 3 for the store page but i need one to show how much the game has improved and since i'm nearly done with it, i'm going to make it the final release trailer (or at least something very much like it).

Those aren't too bad. The cinematic one starts off bit too slow, though. Also, the text in the other one should be much more noticable. If you want to tell the viewer something, don't put it in the upper left corner, where it can be confused with ingame GUI elements. Put it in the middle of the screen. Other than that I'd say you are on the right track. Good luck with your game.